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1 High Frequency Permeability of Magnonic Metamaterials High frequency permeability of magnonic metamaterials with magnetic inclusions of complex shape O. Dmytriiev1,6,a, M. Dvornik1, R. V. Mikhaylovskiy1, M. Franchin2, H. Fangohr2, L. Giovannini3, F. Montoncello3, D. V. Berkov4, E. K. Semenova4, N. L. Gorn4, A. Prabhakar5, and V. V. Kruglyak1,b 1. School of Physics, University of Exeter, Exeter, EX4 4QL, United Kingdom 2. School of Engineering Sciences, University of Southampton, SO17 1BJ, United Kingdom 3. Dipartimento di Fisica, Università di Ferrara, Via G. Saragat 1, 44122 Ferrara, Italy 4. Innovent Technology Development, Pruessingstrasse, 27B, D-07745, Jena, Germany 5. Indian Institute of Technology Madras, Chennai, 600036, India 6. Institute of Magnetism, Kiev, 03142, Ukraine Abstract We present a method of calculation of the effective magnetic permeability of magnonic metamaterials containing periodically arranged magnetic inclusions of arbitrary shapes. The spectrum of spin wave modes confined in the inclusions is fully taken into account. Within the scope of the proposed method, we compare two approaches. The first approach is based on a simple semi-analytical theory that uses the numerically calculated susceptibility tensor of an isolated inclusion as input data. Within the second approach, micromagnetic packages with periodic boundary conditions (PBC) are used to calculate the susceptibility of a single 2D periodic array of such inclusions, with the whole 3D metamaterial consisting of a stack of such arrays. To calculate the susceptibility tensor of an isolated inclusion, we have implemented and compared two different methods: (a) a micromagnetic method, in which we have employed three different micromagnetic packages: the finite element package NMAG and the two finite differences packages OOMMF and MicroMagus; and (b) the modified dynamical matrix method. To illustrate the methodology, we have calculated the effective permeability of a metamaterial consisting of a stack of hexagonal arrays of magnetic nanodisks in a non-magnetic matrix. The range of geometrical parameters for which such a metamaterial is characterized by the negative permeability has been identified. The critical comparison of the different micromagnetic packages and the dynamical matrix method (based on the calculation of the susceptibility tensor of an isolated inclusion) has demonstrated that their results agree to within 3 %. a Also for correspondence: [email protected] b Corresponding author: [email protected] 1 I. INTRODUCTION The recent progress in electromagnetic metamaterials has been fueled by the discovered ability to design their unusual properties1,2 via tweaking the geometry and structure of the constituent “meta-atoms”3. Along with negative permittivity, negative permeability is one of the necessary features for the design of negative refractive index metamaterials. A metamaterial designer can achieve negative permeability via geometrical control of high frequency currents, e.g. in arrays of split ring resonators4, or alternatively can rely on spin resonances in natural magnetic materials5,6, as was suggested by Veselago in Ref. 1. However, the age of nanotechnology sets an intriguing quest for additional benefits to be gained by nano-structuring natural magnetic materials into so called magnonic metamaterials, in which the frequency and strength of resonances based on spin waves (magnons)7 are determined by the geometry and magnetization configuration of meta-atoms. Spin waves can have frequencies up to hundreds of GHz (in the exchange dominated regime)6-9 and have already been shown to play an important role in the high frequency magnetic response of composites containing magnetic inclusions of cylindrical10-12 and spherical13-17 shape. The majority of analytical models of the effective permeability of magnetic composites and metamaterials employ the macrospin approximation, in which each magnetic inclusion within a non-magnetic matrix is considered as a single giant spin and is therefore characterized by a single magnetic resonance. However, it is well known that the spin wave spectrum of magnetic nano-structures and nano-elements has a complex structure, featuring series of resonances due to spatially non-uniform spin wave modes18-22. Each of the resonances is expected to contribute to the susceptibility tensor of the magnetic constituents and correspondingly to the permeability tensor of the whole metamaterial. The resonance frequencies can be controlled and reconfigured by the external magnetic19-24 and electric25,26 fields, and the same functionalities could therefore be inherited by the magnonic metamaterials. In this paper, we demonstrate a method of calculation of the effective permeability that takes full account of the complex spectrum of the metamaterial’s individual magnetic constituents. In this method, the susceptibility tensor of an isolated inclusion is calculated numerically and then used as an input to an analytical expression for the permeability of the whole metamaterial. To find the susceptibility tensor of the isolated inclusion, different approaches have been used. In one of them, we have performed full-scale numerical micromagnetic simulations using three different micromagnetic packages: a finite element based package NMAG27 and two finite difference based packages OOMMF28 and MicroMagus29. In the other approach, the dynamical matrix method, in which the system of linearized equations of motion of magnetic moments is solved to find the normal modes of a system30, has been modified to facilitate the susceptibility calculations. The methods have been applied to a model metamaterial representing an array of magnetic nano-disks embedded into a non-magnetic matrix. In particular, we have been able to determine the region of geometrical parameters of such a metamaterial, in which one of the components of the permeability tensor becomes negative within a certain frequency range. The predictions of the method are compared with calculations based on micromagnetic simulations with the use of periodic boundary conditions (PBCs) and also with macrospin calculations. Furthermore, we use the calculations to compare the different micromagnetic methods in order to evaluate the accuracy to be expected from micromagnetic simulations. 2 In principle, the proposed method could be considered as an extension of the concept from Ref. 6 of using magnonic resonances to tailor effective permeability of metamaterials. However, we note that the stack of thin films studied in Ref. 6 is treatable analytically. In practice however, one might either want or have to deal with alternative realizations of the concept, i.e. to use magnonic “meta-atoms” of a different shape. For example, this could be dictated either by limitations of the available nanofabrication tools or by needs for permeability with a specific frequency dependence. Such more complex magnonic metamaterials would not necessarily allow a simple analytical treatment while numerical simulations of extended samples might present too high demands on computational resources. The proposed method circumvents the problem, to some degree in the spirit of approaches developed in Refs. 31,32. II. PERMEABILITY OF A MAGNONIC METAMATERIAL A. Analytical model Let us consider an idealized case of an infinitely extended 3D metamaterial. To enable a meaningful introduction of the effective permeability ˆ , the wavelength of electromagnetic waves should be much greater than the characteristic dimensions of the magnetic inclusions and the lattice constant of the metamaterial. Then, we can use the standard “macroscopic” relation between the high frequency magnetic induction (B) and magnetic field (H) of the electromagnetic wave: B H 4 M ˆH, (1) where M is the dynamic part of the magnetization that is spatially averaged of the volume of the metamaterial (“macroscopic magnetization”), and the permeability is defined in the frequency domain, ˆ ˆ . We generally denote macroscopic quantities by capital letters and microscopic ones by lower case ones. In particular, the static spatially averaged macroscopic magnetization is denoted as M0. Besides dynamic magnetic field H, there is also external spatially uniform static magnetic field H bias . Permeability ˆ is related to macroscopic susceptibility of the whole metamaterial ˆ as ˆ Iˆ 4 ˆ . (2) Susceptibility ˆ can be found by calculating the response of the volume averaged magnetization of the metamaterial to an external ac uniform magnetic field, which in general is a complicated problem to compute. The problem is simplified by assuming that the metamaterial represents a periodic lattice of magnetic elements (‘inclusions’) that are identical in terms of both their shape and material properties. Then, permeability ˆ of the metamaterial can be related to the susceptibility a single inclusion ( ˆ incl ) via a simple equation. The problem of finding the susceptibility a single inclusion is significantly simpler than that of finding the susceptibility of the whole metamaterial. The two susceptibilities differ due to the dipole-dipole interaction between inclusions inside the metamaterial. If this interaction is absent, ˆ and ˆ incl differ only 3 by a factor equal to volume fraction (“filling factor”) of the magnetic inclusions in the metamaterial (0 1). Indeed, macroscopic magnetization M can be written as M m , (3) where m is the dynamic “microscopic magnetization” (i.e. one obtained via spatial averaging over the volume of a single magnetic inclusion), with the corresponding
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